1. Field of the Invention
This invention relates to a computer readable medium recording a calibration program or the like to adjust the output density of an image formation device, and in particular, relates to a computer readable medium recording a calibration program or the like which is able to accurately detect patch positions during acquisition of calorimetric values from a patch sheet performed in the calibration, to enable execution of calibration with higher precision than in the prior art.
2. Description of the Related Art
In general, ink is dispensed or toner is provided for printing media in a printer or other image formation device based on image data represented by density-gradation values for different colors. Image formation processing is performed on each of the density-gradation values in this image data such that the density (color values) in the image actually formed on the printing media are prescribed reference values (target values). Normally there are differences between individual printers or other image formation devices, and there are also differences between devices in the relations between the above-described density-gradation values and the color values actually output (the density characteristics). Hence at the time of device shipment, color correction information (for example, a color correction table) is determined according to the density characteristic for the device, and during image formation, color correction based on this color correction information is performed for each density-gradation value of the image data used in image formation.
However, when used in such an image formation device, changes with the environment and aging of various portions occur, and changes in the state of the engine and other portions which actually perform image formation on printing media occur. This is accompanied by changes in the above-described density characteristic, and so to maintain the output result at the above-described target values, the initial color correction information must be adjusted appropriately.
Hence in the prior art, calibration of image formation devices has been performed. One such method uses a patch sheet. In this method, for each of the colors of the color material (toner or ink) used in the image formation device, a patch sheet, on which are printed a plurality of patch patterns in which the gradation values of image data are varied, is output. The densities (calorimetric values) of each of the patch patterns on the patch sheet are measured, and the color correction information (for example, a color correction table) is updated such that, for the gradation value of each patch, the difference between the target values and the measured value is corrected.
In calibration using such patch sheets, during color measurements of each of the above-described patch patterns, of course in order to accurately execute calibration it is essential that the color and gradation values at which each of the patch patterns printed on a sheet have been generated be correctly recognized. In particular, when using an inexpensive and simple scanner rather than using precise calorimetric equipment for color measurements of patch patterns, after reading a patch sheet as image data, the positions in the image data of patch patterns printed at different gradation values for different colors must be accurately detected.
As patch pattern position detection methods for this purpose, as for example disclosed in Japanese Patent Laid-open No. 8-9178 and Japanese Patent Laid-open No. 4-87454, methods have been proposed in which marker patches having color/gradation values easily recognized from patch patterns are used, and in which marker patches are first detected, and the positions of patch patterns are detected from the relative positional relations with marker patches.
However, when a scanner is used in color measurements of each patch pattern on a patch sheet, a human sets the patch sheet on the scanner, and so the patch sheet may be tilted or shifted with respect to the prescribed position on the scanner. In this case, when the output from the image formation device is quite thin (light), even when using the above-described marker patch method of the prior art, the marker patch itself, or the position of the marker patch cannot be accurately detected, and detection of the positions of the above-described patch patterns becomes inaccurate.
When position detection becomes inaccurate in this way, the measured density of the patch pattern for some gradation value may be erroneously regarded as a different gradation value, or color measurement may be performed for portions other than patch patterns, and so of course the calibration precision is reduced.